This invention relates to apparatus for gripping and transporting articles during performance of work operations thereon or therewith.
In order to be suitable for use in programmable robots, such apparatus must meet the following design criteria:
1. The gripping surfaces of the fingers must remain parallel to each other throughout their full excursion in order to insure against splaying when handling parts of differing dimensions.
2. The gripping surfaces of the fingers must move rectilinearly toward and away from each other so as to maintain a parallel relationship to the surface of the article being attacked.
3. The gripping force exerted by the fingers must be reasonably constant irrespective of the degree to which the fingers are extended when gripping the article.
4. Tactile and feedback sensing means must be provided so that the positions of the fingers will be known at all times and continuously controllable throughout a sequence of complex movements.
U.S. Pat. No. 3,154,339 discloses a pair of parallel finger beams which, by respective links, are constrained to move opposing fingers toward each other with a substantially constant gripping force. However, the gripping surfaces of the fingers do not remain parallel to each other and do not move rectilinearly during their full excursion, and no sensing means is provided. Hence, this patented arrangement fails to meet criteria 1, 2 and 4 above.
Russian Pat. No. 568,542 discloses an arrangement employing a pair of parallel finger beams and associated links for controlling movement of opposing fingers toward and away from each other. With this linkage arrangement, as the finger beams are moved under piston control to displace the fingers toward each other, the amount of gripping force exertable by the fingers does not remain constant; in fact, it can be shown mathematically that the gripping force progressively reduces toward zero as the thickness of the gripped article diminishes. This requires the use of thick pads on the fingers (as shown in phantom in FIG. 1 of this application) if thin articles are to be gripped. Also no sensing means is or can be embodied in this arrangement to provide servo control because of the non-linear relationship between movement of the piston and movement of the gripping fingers. This configuration therefore fails to meet criteria 3 and 4 above.
The Proceedings of the Fifth International Symposium of Industrial Robots, held at the IIT Research Institute on Sept. 22-24, 1975 discloses a number of gripper-type mechanisms. Only one of these, numbered "12" on page 335, is of interest. This mechanism (which is reproduced as FIG. 2 of this application) comprises two sector gears a and three gear racks b and two linear bearings c. This configuration meets all the criteria except 4 above. However, it constitutes a costly, complex mechanism unsuitable for robots because the two gear racks attached to the fingers must be guided by costly linear ball bearings having an aspect ratio of 2:1. As a result, with a finger excursion stroke of eight centimeters, the length of the guide must be approximately three times the stroke or 24 centimeters long. Thus the linear bearings which are needed to support the racks get in the way of the fingers and restrict their mobility and accessibility to close-in work areas. These linear bearings extend outwardly to each side of the fingers and, by analogy, could be considered equivalent to a person, while grasping a ruler wider than the width of his hand, attempting to get into a tight work area.
There is a need for a relatively simple gripper apparatus suitable for programmable robots and meeting all four criteria without interfering with the ability of the fingers to operate effectively in confined spaces.